In general, a radio frequency amplifier circuit has a configuration in which unit transistors are connected in parallel for the purpose of achieving a high power. As a result, the input and output impedances are low. For example, when the output exceeds 100 W, the input and output impedances are sometimes 3Ω or lower. Resulting impedance mismatching with an external circuit normalized to 50Ω to which the radio frequency amplifier circuit is to be connected requires a matching circuit to be connected to an input side and an output side of the radio frequency amplifier circuit.
The matching circuit for matching the low impedance of the nearly short-circuited radio frequency amplifier circuit to 50Ω is configured, for example, by the combination of an inductive element having wires and distributed constant lines connected in series with a signal path and a capacitive element having capacitors connected in parallel with the signal path.
Conventional matching circuits have configurations as illustrated in Patent Literatures (PTLs) 1 and 2, for example.
FIG. 8 illustrates the configuration inside a package of the conventional high power amplifier described in PTL 1. The high power amplifier described in this figure includes a transistor 801, an input-side matching circuit 802 and an output-side matching circuit 803. The input-side matching circuit 802 and the output-side matching circuit 803 each have a lead terminal 804. The lead terminal 804 takes out a signal from each matching circuit to an outside of a package frame 805.
The input-side matching circuit 802 has a configuration in which a parallel circuit including resistors 806 and a capacitor 807 and a distributed constant line are connected in series and the parallel circuit and the distributed constant line are further connected to a lead terminal. This configuration can stabilize the circuit, thus preventing oscillations. Also, the configuration in which the plural resistors 806 are provided for the capacitor 807 allows the resistance value to be varied according to the connection positions of wires, making it possible to adjust circuit characteristics easily after mounting.
On the other hand, the output-side matching circuit 803 includes a first distributed constant line, a second distributed constant line and the lead terminal 804 in this order from the vicinity of the transistor 801. It is noted that the distributed constant lines may be operated as a parallel plate capacitor.
FIGS. 9A and 9B respectively illustrate the configuration of a conventional radio frequency amplifier described in PTL 2. The radio frequency amplifier described in FIG. 9A includes a transistor 901, an input-side matching circuit 902 and an output-side matching circuit 903A. The output-side matching circuit 903A has a configuration in which distributed constant lines 904 and 906 constituted by high-dielectric substrates and a distributed constant line 905 constituted by a dielectric substrate different from the high-dielectric substrates are cascaded using wires. Similarly to the output-side matching circuit 803 in FIG. 8, for the transformation from a low impedance to a high impedance, the wires and the distributed constant lines also functioning as the parallel plate capacitor are constituted in three stages, thereby ensuring an electrical length. In the case where the high-dielectric substrates constituting the distributed constant lines 904 and 906 are the same in relative dielectric constant and thickness, the distributed constant lines 904 and 906 can be formed of a single high-dielectric substrate 910 as shown in FIG. 9B. Part of the structural components of the output-side matching circuit 903A in FIG. 9A can be formed on the single substrate as those of the output-side matching circuit 903B in FIG. 9B, making it possible to reduce the longitudinal size of the matching circuit without changing the configuration thereof.